Should you buy the iPhone 6 or iPhone 6 Plus, buy one of the older iPhones, wait for the iPhone 7 or keep the one you have? Here's our buying advice for prospective iPhone buyers.

The iPhone 6, on the other hand, offers a 1334-x-750-pixel LED-backlit display. Despite the fact that the display measures 4.7-inch, it works out to 326 pixels per inch, which is the same as the iPhone 5 and 5s.

Apple calls these new iPhones Retina HD display even though only the Plus offers a greater number of pixels per inch. in fact it offers two million pixels in total, that's nearly a million more pixels than the iPhone 6. However, both the new displays feature an LCD with dual-domain pixels for improved viewing angles and a substantially higher contrast ratio, so the iPhone 6 screen is still better than the iPhone 5s.

It's the contrast ratio that provides the biggest difference: last year's flagship iPhone 5s had an 800:1 ratio. This year the iPhone 6 has a 1400:1 ratio, and the larger 6 Plus offers 1300:1.

The iPhone 6 Plus offers 88% more viewing area and three times the pixels on iPhone 5/5s.

It sounds impressive, but remember, while the iPhone 6 Plus packs a million more pixels into the display, but there comes a point where we can't actually distinguish any difference — these displays are referred to as Retina Displays because there are only so many pixels your eye can see.

iPhone 6 Plus screen:

Retina HD display

5.5-inch display (compared to 4-inch)

1920-by-1080 pixel resolution at 401ppi

1300:1 contrast ratio

Dual-domain pixels for wider viewing angles

iPhone 6 screen:

Retina HD display

4.7-inch display (compared to 4-inch)

1334-by-750 pixel resolution at 326ppi

1400:1 contrast ratio

Dual-domain pixels for wider viewing angles

iPhone buying guide: What storage capacity to go for, and how many GB is best for an iPhone

Prior to the launch of the iPhone 6, iPhones tended to come in 16GB and 32GB versions, with a 64GB version available for the top of the range model. Apple has also been known to offer an 8GB on the most affordable iPhone. Read about why it's not a good idea to buy the 16GB version: The problem with Apple's 16GB iPhones and iPads

The current line up is slightly different:

The iPhone 6 and iPhone 6 Plus are available in 16GB, 64GB, and 128GB.

The iPhone 5s comes in 16GB and 32GB.

And the iPhone 5c comes in 8GB.

How do you decide how many gigabytes are necessary?

Apps are getting larger, and photos and videos are addictive to shoot and gobble up and ever-increasing amount of gigabytes. For that reason, as we mention above, we would always recommend going for as many GB as you can afford.

If you already have an iPhone check how much space all your apps and data are eating up on your by looking at Settings > General > Usage.